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Power Operated and Colour Light Signals

Power Operated Mechanical Signals

Signalling using electrical systems appeared on the Lancaster & Preston Railway as far back as 1849 but the equipment available at that time was simply not reliable enough for the job. Power operated points and semaphore signals started to appear in about 1900. There were various systems used but the most common point operating system was probably the electro-hydraulic type. In these the signalman operated a switch which caused a hydraulic pump at the point to move the blades across.

On the subject of power operated semaphore signals John Webb was able to advise that-
There were many schemes in the 20th century (Crewe, Hull, LSWR in the Southampton area, Southern Railway after 1923, East coast mainline around Thirsk etc) where some form of power signalling was in use. They included electrical solenoids, electro-pnumatic, electric motor etc. I warmly recommend "Two Centuries of Signalling History" by Geoffery Kitchenside and Alan Williams, published by OPC in 1998 for a quite detailed and well-illustrated tour of signalling developments.

The first power operated signal box in Britain was at Bishopsgate in London, on the Great Eastern Railway, which opened in 1899 (the first in the world was probably the Westinghouse system installed on the Philadelphia & Reading Railroad in America in 1884). The Lancashire & Yorkshire Railway was one of the pioneers of power operation and they also developed double-acting three position levers for power operated signal boxes. In the centre position everything was at its normal orientation, pulling the lever set a series of points for one route, pushing it set them the other way. The combination of power operation and three-position switches resulted in a dramatic reduction in the number of levers required for a given signal box but the system was not widely adopted.

Power operation also reduced the number of signal boxes required as it allowed the box to operate signals at much greater distances. The re-signalling of a station at Wigan in 1939 with colour light signals and power operated points reduced the signals boxes required from twelve to three.

Electric Coloured Light Signals

Electric coloured light signals were experimented with as early as 1913 but it was the 1920's before they became widespread. The lamps used need to have six or seven thousand candle power to be clearly visible in daylight and making lamps which were sufficiently powerful and also reliable was a major challenge for the existing technology.

Incidentally as it is railway terminology to refer to a signal as 'on' when it is at stop or red and 'off' when it is at proceed, i.e. yellow or green confusion is avoided by not using on and off to refer to individual lamps they would be referred to as 'lit' and 'out'

Early colour light signal designs were many and varied, the light units were usually mounted on a post and pointing along the track. Mounted on the front of the light housing was a large back-plate painted black to make the signal more visible. Building on trusted technology the signals had a single lamp with a reflector behind and a mechanical spectacle plate carrying coloured glass filters mounted in front of the lamp, all inside the casing. The spectacle plate was operated by polarized relays, switching between red and either green (for a home signal) or yellow (for a distant signal). This single glass design was widely used and remained popular on Eastern Region of BR for many years. They are still in widespread use around the world, notably in the USA.

More recent designs have favoured the use of separate lamps and lenses for the different colours. The simplest type is the two aspect signal which has two separate lights, one above the other. These can be used to replace simple semaphore signals with red and green lights for a 'home' signal and red and yellow for a 'distant'. The first serious use of colour light signals was on the Liverpool Overhead Railway where over fifty two-aspect (that means two lens) colour light signals were installed in 1921. The three aspect colour light signal has three lights, red at the bottom then yellow and green at the top. The three aspect signal serves the same function as the combined home and distant semaphore type signal as shown in the sketch below.

Fig___ Early colour light signals

Sketch showing Early colour light signals

Note you will often see colour light signals with long 'hoods' on the front, these were introduced in the Second World War as part of the black-out precautions to prevent enemy bombers using the signals to find their way to the target. Once fitted they remained in place until major maintenance was required and some long hoods lasted into the 1970's.

The most common design on busy lines is the four-aspect signal which has four separate lamps mounted one above the other. The four aspect has a yellow at the top, then a green, another yellow and a red at the bottom. Red means stop, green means all-clear, yellow means the next signal is red and double yellow means the next signal is yellow. The four aspect signal first appeared on the Southern Railway commuter lines in the London area in 1923 and its application is probably best explained in the form of a drawing.

Fig___ Four aspect colour light signalling

Sketch showing operation of Four aspect colour light signalling

Junction signals have a rack of five white lamps inclined at 45 degrees in the direction of the diverging line. These lights are called 'feathers' by railwaymen and several sets of 'feathers' can be added at thirty degree angles where more than one diverging route is approached. Adding the 'feather' to a simple two aspect signal means it can replace a bracket signal with two arms. When the signal shows a red light it means 'Stop', a green light with the feather switched off means 'Proceed on the main line', a green light with the white feather illuminated means 'Proceed on the branch line'. A three aspect signal with a feather can replace four semaphore arms on a bracket signal, in this case a yellow light with no feather means 'Proceed on the main line with caution' and a yellow with the feather illuminated means 'Proceed on the branch line with caution'. It is quite usual to see feather added to four aspect signals, particularly at junctions on busy suburban or main lines.

Fig___ Junction signal with semaphore equivalent

Sketch showing colour sequence for colour light Junction signal with semaphore equivalent

There are two different variations on the old mechanical route indicating box. The stencil type has a fixed cut-out with a light behind it, these boxes are typically twelve inches wide by nine inches high by about six inches deep and you may see two or even three of these small boxes one above the other. They are usually associated with ground signals, allowing one signal to control a number of sidings. The second design, known by railwaymen as 'theatre' indicators, uses a box with an array of lights which can be switched to show letters and numbers as required, these tend to be quite large, typically eighteen inches square by six inches thick. At Stockport station, which is on a four-track main line, the platform starter signals have this latter type. The lights are switched to show F or S for Fast and Slow lines. Originally the theatre indicators used a matrix of lamps, usually 5 x 7, but more recently fibre optics have been used, allowing a single lamp to be used for each indication.

The modern colour light ground signal has three lights, two white and a red arranged as shown in the sketch, the lower white light is always on. Two white lights means 'all clear', one white and a red means 'stop'. To avoid having multiple electric ground signals it is common practice to add a stencil route indicator allowing one signal to cover two or three routes.

The old 'shunt ahead' and 'calling on' signals described above have been replaced with small units resembling modern electric colour-light ground signals but with only two lights, both white. These units are mounted on the post or gantry immediately below the main signal unit and often have hoods on the lenses to make them more visible against the sky.

Fig___ Sketches of modern signals

Sketches of modern railway colour light signals

Models of colour light signals are available from a number of sources, most now use LED's but I believe some still use small 'grain of wheat lamps'. These lamps are less reliable than LED's, running them on reduced voltage prologues their life considerably but changing the bulbs when they fail calls for a steady hand (and a removable signal!). Both these ranges offer two, three and four aspect signals suitable for any period from the later 1920's to the present day.

The colour light equivalent of the old banner repeater signal is a circular housing with a matrix of white lights. These are switched to show the position of the associated signal. John Sullivan was kind enough to supply me with a photograph of one of these units in use in 2003, the picture on the left shows the signal displaying the all clear indication. The photo on the right is a free standing banner repeater with an associated stencil display in use at Stockport in 2007. This signal is located at the end of a platform road close by an overbridge where the main line curves slightly, so the actual signal cannot be seen, it is displaying the 'danger' aspect.

Fig___ Colour light banner repeater signal

Photos showing a Colour light banner repeater signal

A photo of the older mechanical type is included in the section on Signals - Mechanical Types & Fog Signals.

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